The invention relates to a hydraulic system for moving an implement of a working machine, a working machine, in particular a wheel loader, comprising a hydraulic system, a method for moving an implement of a working machine, a computer program, a computer readable medium, and a controller for a hydraulic system.
A working machine, such as a wheel loader, is usually provided with a bucket, container, gripper or other type of implement for digging, carrying and/or transporting a load. For example, a wheel loader has a lift arm unit for raising and lowering the implement. Usually a pair of hydraulic cylinders is arranged for raising the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm.
In addition, the working machine is often articulated frame-steered and has a pair of hydraulic cylinders for turning/steering the working machine by pivoting a front section and a rear section of the working machine relative to each other.
The hydraulic system generally further comprises at least one hydraulic pump, which is arranged to supply hydraulic power, i.e. hydraulic flow and/or hydraulic pressure, to the hydraulic cylinders. In traditional wheel loaders, the hydraulic pump is driven by the internal combustion engine of the working machine. In additional, the hydraulic system of a working machine is usually a so-called load sensing system (LS-system). This means that the pump receives a signal representing the current load pressure of a hydraulic cylinder in operation. The pump is thereby controlled to provide a pressure which is somewhat higher than the load pressure of the cylinder.
With increasing demands for more energy efficient working machines, traditional systems for powering the hydraulic cylinders present certain problems. For example, LS-systems require a pressure drop for the hydraulic cylinder control, and this requires the pump to provide more energy than what is required for the hydraulic cylinders to perform their respective tasks. Thus, there is a desire to provide a more energy efficient solution for working machine hydraulic systems.
It is desirable to reduce energy consumption for moving implements of working machines.
According to an aspect of the present invention, a hydraulic system for moving an implement of a working machine is provided,
the hydraulic system comprising a hydraulic cylinder with a cylinder and a piston which is adapted to move in the cylinder to thereby move the implement relative to a body structure of the working machine, and an actuator pump arranged to provide hydraulic fluid to the hydraulic cylinder, the hydraulic cylinder having a first port and a second port adapted to be in fluid communication with the actuator pump,
the hydraulic cylinder and the actuator pump being arranged so that the hydraulic cylinder is directly controlled by the actuator pump so that the rate of movement of the piston of the hydraulic cylinder is purely pump controlled,
the hydraulic system further comprising a hydraulic accumulator for suspension of the implement, which hydraulic accumulator is arranged to be selectively connectable to the first port,
the hydraulic system further comprising a further pump in addition to the actuator pump, the hydraulic accumulator being arranged to be pressurised by the further pump.
It is understood that the actuator pump and the further pump are hydraulic pumps. It is further understood that the movement of the hydraulic cylinder piston provides for the hydraulic cylinder to change length to thereby move the implement relative to a body structure of the working machine.
The hydraulic cylinder may be a lifting hydraulic cylinder adapted to raise and lower the implement relative to the body structure of the working machine. For example, the implement may be arranged on an elongated load arm, also referred to as a boom, for lifting and lowering the implement relative to the body structure. The elongated load arm may be at a first end pivotally connected to the body structure, and the implement may be mounted to the load arm at a second end of the load arm. The lifting hydraulic cylinder may extend between the body structure and the load arm. Thus, the lifting hydraulic cylinder may provide for lifting the implement by a pivoting movement of the load arm around its first end.
The hydraulic cylinder may alternatively be a tilting hydraulic cylinder adapted to tilt the implement relative to the body structure of the working machine. For example, the implement may be pivotally mounted to the load arm at the second end of the load arm, and the tilting hydraulic cylinder may extend from the load arm or the body structure to a linkage mechanism, which is adapted to transfer movements from the tilting hydraulic cylinder to the implement to tilt the implement.
The hydraulic cylinder and the actuator pump being arranged so that the hydraulic cylinder is directly controlled by the actuator pump so that the rate of movement of the piston of the hydraulic cylinder is purely pump controlled, means that the hydraulic cylinder is flow controlled. This means that the rate of movement of the piston is directly proportional to the fluid flow generated by, and therefore passing through the actuator pump. Thus, the hydraulic cylinder and the actuator pump may be connectable directly to each other. Thereby, the rate of movement of the piston of the hydraulic cylinder may be controlled solely by the actuator pump, or solely by the actuator pump and a boost pump as exemplified below. There may be a linear relationship between the rate of movement of the piston of the hydraulic cylinder and the fluid flow generated by the actuator pump.
Controlling the rate of movement of the piston of the hydraulic cylinder is herein understood as not including changing the direction of movement of the piston within the hydraulic cylinder. However, as exemplified herein, the actuator pump may nevertheless be arranged so as to provide a change of the piston movement direction, e.g. in the case of a rotational pump, by changing the pump rotation direction. Nevertheless, such a movement direction change may also be provided by a suitable valve arrangement. In any case, the rate of movement of the piston is purely pump controlled. Thus, the hydraulic cylinder control does not include controlling the rate of movement of the piston with a valve. I.e. the change of the piston velocity from one velocity in one of the two directions in the cylinder, to another velocity in the same direction in the cylinder, is purely pump controlled.
It is understood that the hydraulic cylinder will normally be influenced by the force of gravity, and a pure pump control may include power being provided to the hydraulic cylinder from the actuator pump, or power being delivered to from the hydraulic cylinder to the actuator pump, e.g. in the case of energy recuperation, as exemplified below. In the latter case, a movement of the piston, although caused by gravity, is understood here as being purely pump controlled, e.g. by the control of a braking torque of the pump.
It is understood that the rate of movement of the piston of the hydraulic cylinder is equal to the rate of change of the length of the hydraulic cylinder. It is further understood that by changing the length of the hydraulic cylinder, it is extended or shortened.
The fluid flow generated by the actuator pump may be controlled by controlling the displacement of the actuator pump or the speed of the actuator pump. Such fluid flow control may, in cases of pump speed control, be accomplished by the actuator pump being a rotational pump and by control of the rotational speed of the pump. In other embodiments, where the actuator pump has a variable displacement, the fluid flow control may be accomplished by control of the displacement setting of the pump.
The direct proportionality of the rate of movement of the piston of the hydraulic cylinder to the fluid flow generated by the actuator pump is preferably utilised so that the actuator pump speed and/or displacement is the single control variable of a control unit for the hydraulic cylinder. This in turn means, as opposed to LS-systems, no valve arrangement between the pump and the hydraulic cylinder is needed for the hydraulic cylinder control. Thus, no pressure drop in the system is required for the hydraulic cylinder control. In turn, this will allow the actuator pump to work, compared to a pump in an LS-system, with reduced power for a given task of the hydraulic cylinder. This will reduce energy consumption of the working machine implement manipulation.
The hydraulic accumulator may be arranged to be selectively connectable, e.g. with a valve, to the first port to be in free fluid communication with the first port. The hydraulic accumulator adapted to be in free fluid communication with the first port will provide, for example when the working machine is driven with the implement loaded, flexibility between the body structure and the implement, which is turn will smoothen the ride of the working machine, e.g. by absorbing shocks where the ground is rough. It is understood that the free fluid communication between the hydraulic accumulator and the first port allows fluid to flow freely in the connection between the hydraulic accumulator and the first port. Thereby, the hydraulic accumulator will provide an absorption of movements, e.g. oscillations, of the implement in relation to the body structure.
Some versions of implement suspension functions are known per se. Where the hydraulic cylinder is a lifting hydraulic cylinder adapted to raise and lower the implement, the arrangement of a hydraulic accumulator connectable to the hydraulic cylinder may be referred to as a boom suspension system (BSS). However, using the same pump for actuation and hydraulic accumulator charging will create a lack of accuracy in the hydraulic cylinder control. The reason is that the direct proportionality of the rate of movement of the piston of the hydraulic cylinder to the fluid flow generated by the actuator pump might be utilised for the hydraulic cylinder control, and if the actuator pump is not utilised solely for powering the hydraulic cylinder, it will not be possible to correctly determine the rate of movement of the piston of the hydraulic cylinder based on the fluid flow generated by the actuator pump.
Since the hydraulic accumulator is adapted to be pressurised by a further pump which is provided in addition to the actuator pump, the actuator pump can be dedicated only to power the hydraulic cylinder. This will in all operational situations make it possible to correctly determine the rate of movement of the piston of the hydraulic cylinder based on the fluid flow generated by the actuator pump.
The further pump may be any suitable pump in the working machine, which is provided in addition to the actuation pump, e.g. a pump for a hydraulic steering system of the working machine, for a hydraulic brake system of the working machine, and/or for a cooling fan of the working machine.
The first port of the hydraulic actuator may be provided on a piston side of the piston, i.e. the side without a piston rod, and the second port may be provided on a piston rod side of the piston. The first second ports may be adapted to be in fluid communication with respective ports of the actuator pump.
It is understood that by the hydraulic cylinder presenting the first and second ports adapted to be in fluid communication with the actuator pump, the hydraulic cylinder is adapted to move the implement in response to hydraulic fluid from the actuator pump being selectively directed to the first and second ports so as to move the hydraulic cylinder piston to change the length of the hydraulic cylinder. The possibility to select the fluid direction might be accomplished by a suitable valve arrangement, or by pump direction control, as exemplified below.
Preferably, the hydraulic cylinder and the actuator pump are arranged so that when the piston in the hydraulic cylinder is moved, fluid is moved from one of the first and second ports towards the other of the first and second ports via the actuator pump. Thus, the actuator pump may be provided as a bi-directional pump, which operates by merely moving fluid from one side of the hydraulic cylinder piston to another side of it. This provides a simple and robust solution.
Preferably, the hydraulic cylinder is a lifting hydraulic cylinder adapted to raise and lower the implement relative to the body structure of the working machine. Preferably, the actuator pump is adapted to be powered by an electric machine, in the form of an electric motor and generator, i.e. a device which can work as a motor as well as a generator. Preferably, the hydraulic system comprises an electric energy storage arrangement, and the electric machine is adapted to be electrically connected to the electric energy storage arrangement, the electric machine being adapted to be driven by the actuator pump when the implement is lowered relative to the body structure, and to thereby provide a charging current to the electric energy storage arrangement. Thereby, at least a part of the energy used for raising the implement may be recovered when lowering the implement. Such energy recuperation using the actuator pump will further increase the energy efficiency of the working machine.
Preferably, the hydraulic system comprises a boost pump adapted to provide pressurised fluid to one of the first and second ports, so that during extension of the hydraulic cylinder, pressurised fluid is provided from the actuator pump as well as the boost pump. This is particularly advantageous where the hydraulic cylinder and the actuator pump are arranged so that when the piston in the hydraulic cylinder is moved, fluid is moved from one of the first and second ports towards the other of the first and second ports via the actuator pump. For example, where the hydraulic cylinder is a lifting hydraulic cylinder adapted to raise and lower the implement, during extension of the lifting hydraulic cylinder so as to raise the implement, the piston rod therein will provide for less fluid leaving the lifting hydraulic cylinder than fluid needed to enter the lifting hydraulic cylinder. The boost pump will compensate for the effect of the difference in effective pressure area on opposite sides of the piston in the lifting hydraulic cylinder. It is understood that the boost pump is a suitable hydraulic pump.
It is understood that regardless whether or not the system comprises a boost pump, the hydraulic cylinder is flow controlled. More specifically, even if a boost pump is present as described above, the hydraulic cylinder is directly controlled by the actuator pump so that the rate of movement of the piston of the hydraulic cylinder is directly proportional to the fluid flow generated by the actuator pump. In the control of the hydraulic cylinder, the involvement of the boost pump may be taken into account by the volume, and hence the flow, compensated for by the boost pump being known. Thereby, the difference, depending on the direction of hydraulic cylinder movement, in the proportionality between the actuator pump fluid flow and the movement of the hydraulic cylinder piston, is known as well, and can be taken into account in the hydraulic cylinder control.
Where the hydraulic cylinder is a lifting hydraulic cylinder adapted to raise and lower the implement relative to the body structure of the working machine, the first and second ports are herein referred to as first and second lifting ports, and the actuator pump is referred to as a lifting actuator pump.
The hydraulic system may comprise a tilting actuator pump, and a tilting hydraulic cylinder with a cylinder and a piston which is adapted to move in the cylinder to thereby tilt the implement relative to the body structure, the tilting hydraulic cylinder presenting a first tilting port and a second tilting port adapted to be in fluid communication with the tilting actuator pump, the tilting hydraulic cylinder and the tilting actuator pump being arranged so that the rate of movement of the piston of the tilting hydraulic cylinder is purely pump controlled. Preferably, the rate of movement of the piston directly proportional to the fluid flow generated by the tilting actuator pump.
Thus, the direct proportionality of the rate of movement of the piston of the tilting hydraulic cylinder to the fluid flow generated by the tilting actuator pump, may be utilised so that the fluid flow generated by the tilting actuator pump is the single control variable of a control unit for the tilting hydraulic cylinder. This in turn means that, as opposed to an LS-system, no pressure drop in the system is required for the tilting hydraulic cylinder control, which allows the tilting actuator pump to work with an effective power consumption for a given task of the tilting hydraulic cylinder. This will further reduce energy consumption of the working machine implement manipulation. It is understood that the tilting actuator pump is a hydraulic pump.
The first and second tilting ports may be adapted to be in fluid communication with respective ports of the tilting actuator pump. The tilting hydraulic cylinder and the tilting actuator pump may be arranged so that when the piston in the tilting hydraulic cylinder is moved, fluid is moved from one of the first and second tilting ports towards the other of the first and second tilting ports via the tilting actuator pump. Thus, the tilting actuator pump may be provided as a bi-directional pump, providing a simple and robust solution.
Where a boost pump is provided as described above, the boost pump may be adapted to provide pressurised fluid to one of the first and second tilting ports, so that during extension of the tilting hydraulic cylinder, pressurised fluid is provided from the tilting actuator pump as well as the boost pump. Thus, the lifting and tilting hydraulic cylinders may share a single boost pump. This simplifies the hydraulic system, and reduces cost thereof.
It should be noted that in the case of a lifting hydraulic cylinder, it is normally arranged so that it is extended to raise the implement, and if a boost pump is provided for the lifting hydraulic cylinder, it will be arrange to deliver fluid to the cylinder during such raising of the implement. It is however conceivable to provide an opposite arrangement, i.e. where the lifting hydraulic cylinder is arranged, e.g. by some suitable linkage, so that it is shortened to raise the implement, and thereby the boost pump will be arrange to deliver fluid to the cylinder during lowering of the implement.
It should also be noted that by providing a hydraulic cylinder which presents during extension or shortening the same change of volume on both sides of the piston, no boost pump would be needed for such a hydraulic cylinder.
The hydraulic accumulator may be arranged to be selectively connectable to the first tilting port. Thereby, the hydraulic accumulator may be arranged to be in free fluid communication with the first tilting port, which may provide, when the working machine is driven with the implement loaded, a degree of flexibility of tilting movements of the implement, which is turn may smoothen the ride of the working machine.
Where the system comprises a lifting hydraulic cylinder as well as a tilting hydraulic cylinder, the hydraulic accumulator may be arranged to be selectively connectable to the first tilting port and/or the first lifting port. In any case, the hydraulic system comprises a further pump in addition to the actuator pump(s), and the hydraulic accumulator is arranged to be pressurised by the further pump.
According to another aspect of the invention, a method is provided for moving an implement of a working machine comprising a hydraulic cylinder with a cylinder and a piston which is adapted to move in the cylinder to change the length of the hydraulic cylinder to move the implement, the hydraulic cylinder presenting a first port and a second port adapted to be in fluid communication with an actuator pump, the working machine further comprising a hydraulic accumulator for suspension of the implement, which hydraulic accumulator is arranged to be in free fluid communication with the first port. The method comprises
disconnecting the hydraulic accumulator from the first port,
moving fluid to the second port via the actuator pump, whereby the rate of movement of the piston of the hydraulic cylinder is purely pump controlled, so as to shorten the hydraulic cylinder to lower the implement relative to a body structure of the working machine,
pressurizing the hydraulic accumulator by a further pump which is provided in addition to the actuator pump,
moving fluid to the first port by means of the actuator pump, whereby the rate of movement of the piston of the hydraulic cylinder is purely pump controlled, so as to extend the hydraulic cylinder to raise the implement relative to the body structure, and
determining whether to provide a free fluid communication between the hydraulic accumulator and the first port.
The first and second ports may be adapted to be in fluid communication with respective ports of an actuator pump, and moving fluid to the second port may comprise moving fluid from the first port towards the second port. Moving fluid to the first port may comprise moving fluid from the second port towards the first port. The rate of movement of the piston of the hydraulic cylinder may be purely pump controlled such that said rate is directly proportional to the fluid flow through the actuator pump. Said rate may be directly proportional to the fluid flow generated by the actuator pump.
Similarly to the hydraulic system, the method provides for the actuator pump to be dedicated only to power the hydraulic cylinder, since the hydraulic accumulator is adapted to be pressurised by a further pump which is provided in addition to the actuator pump. This will in all operational situations make it possible to correctly determine the rate of movement of the piston of the hydraulic cylinder based on the fluid flow generated by the actuator pump.
Preferably, the method comprises determining the pressure at the hydraulic accumulator and/or at the first port. Thereby, the step of determining whether to provide a free fluid communication between the hydraulic accumulator and the first port, may be based on said determination of the pressure at the hydraulic accumulator and/or the first port. Also, the step of pressurizing the hydraulic accumulator by the further pump, may be preceded by a decision, e.g. by a control unit, whether to pressurise the hydraulic accumulator. Where the further pump is arranged to provide fluid to other consumers in the working machine, determining the pressure at the hydraulic accumulator and/or the first port will provide a possibility to prioritize and/or distribute the further pump work between the consumers, and thereby provide a basis for the decision whether to pressurise the hydraulic accumulator.
Preferably, the method comprises determining whether the pressure at the hydraulic accumulator is at least as high as the pressure at the first port, and the step of determining whether to provide a free fluid communication between the hydraulic accumulator and the first port, is based on said determination whether the pressure at the hydraulic accumulator is at least as high as the pressure at the first port. Thereby, movements of the implement due to a pressure difference between the hydraulic accumulator and the first port, at engagement of the hydraulic accumulator to the first port, can be avoided.
Preferably, the method comprises providing the free fluid communication between the hydraulic accumulator and the first port at least on the condition that the pressure at the hydraulic accumulator is at least as high as the pressure at the first port. Thereby, a sudden drop of the implement, at engagement of the hydraulic accumulator to the first port, can be avoided.
Preferably, providing the free fluid communication between the hydraulic accumulator and the first lifting port comprises allowing fluid to flow freely in the fluid communication between the hydraulic accumulator to the first lifting port. Thereby, the hydraulic accumulator will provide an absorption of movements, e.g. oscillations, of the implement in relation to the body structure.
Preferably, raising the implement comprises powering the actuator pump by an electric machine which is connected to an electric energy storage arrangement, and lowering the implement comprises driving the electric machine by the actuator pump, and thereby providing a charging current to the electric energy storage arrangement.
Preferably, raising the implement comprises providing pressurised fluid from the actuator pump as well as a boost pump.
According to another aspect of the invention, a computer program is provided comprising program code means for performing the steps of the method for moving an implement of a working machine as claimed or described herein, when said program is run on a computer.
According to another aspect of the invention, a computer readable medium is provided carrying a computer program comprising program code means for performing the steps of the method for moving an implement of a working machine as claimed or described herein, when said program is run on a computer.
According to another aspect of the invention, a control unit is provided for a hydraulic system for moving an implement of a working machine, the working machine comprising a hydraulic cylinder with a cylinder and a piston which is adapted to move in the cylinder to change the length of the hydraulic cylinder to move the implement, the hydraulic cylinder presenting a first port and a second port adapted to be in fluid communication with respective ports of an actuator pump, the working machine further comprising a hydraulic accumulator for suspension of the implement, which hydraulic accumulator is arranged to be in free fluid communication with the first port, the control unit being configured to
to control a suspension control valve to disconnect the hydraulic accumulator from the first port,
to control the actuator pump so as to move fluid to the second port via the actuator pump, whereby the rate of movement of the piston of the hydraulic cylinder is purely pump controlled, so as to shorten the hydraulic cylinder to lower the implement relative to a body structure of the working machine,
to control a further pump which is provided in addition to the actuator pump so as to pressurise the hydraulic accumulator,
to control the actuator pump so as to move fluid to the first port by means of the actuator pump, whereby the rate of movement of the piston of the hydraulic cylinder is purely pump controlled, so as to extend the hydraulic cylinder to raise the implement relative to the body structure, and
to determine whether to provide a free fluid communication between the hydraulic accumulator and the first lifting actuator port.
Preferably the control unit is further adapted to determine whether the pressure at the hydraulic accumulator is at least as high as the pressure at the first lifting actuator port, and to determine whether to provide a free fluid communication between the hydraulic accumulator and the first port, based on said determination whether the pressure at the hydraulic accumulator is at least as high as the pressure at the first port.
Preferably the control unit is further adapted to control the suspension control valve, to provide the free fluid communication between the hydraulic accumulator and the first port, at least on the condition that the pressure at the hydraulic accumulator is at least as high as the pressure at the first port.
Preferably the control of the actuator pump to raise the implement comprises control of an electric machine to power the actuator pump, which electric machine is connected to an electric energy storage arrangement, and the control of the actuator pump to lower the implement comprises control of the electric machine to be driven by the actuator pump so as to provide a charging current to the electric energy storage arrangement.
Preferably, where the hydraulic system comprises a boost pump, the control unit is adapted to control the boost pump as well as the actuator pump to provide pressurised fluid from the actuator pump as well as the boost pump when raising the implement.